The present invention relates in general to backscatter communication performed by a mobile unit powered by an RF harvesting power supply, and, more specifically, to vehicle communication for ensuring an RF environment sufficient to maintain adequate power from the RF harvesting power supply.
Backscatter uses the reflection of incident radio signals as a means of communication. Devices that backscatter RF generally require very little power (e.g., a few microwatts to tens of microwatts) and can actually be powered by incident radio signals in the local RF environment (including the RF signal being backscatter and/or other incident RF radiation). This enables RF identification (RFID) tags that do not require a battery.
In some types of RFID systems employing backscatter, an active reader transmits a signal to a battery-less tag. Some tags may have the capability to convert the frequency of the transmitted signal to a different backscattered frequency. The tag uses the energy of the incoming signal to power a controller that alternates an impedance of an antenna to modulate the signal and reflect the signal back with new information that can be decoded by the reader.
Passenger vehicles typically employ a remote keyless entry (RKE) system wherein a wireless key fob carried by a user communicates with an RKE receiver in the vehicle to provide remote user access to functions such as locking and unlocking of doors and trunk, powered opening and closing of liftgates, car finder, panic alarm, activation of lights, and remote engine start. Known key fobs employ active transmitters which require batteries. It would be desirable to eliminate batteries to provide more convenience to the user, improved structural robustness, and lower cost.
Key fobs may include devices carried by the vehicle user (e.g., in a pocket or purse) as well as keypad units that are attached to the vehicle exterior which work via RF without direct connection to the vehicle electrical system. The invention also applies to other RF signaling systems in a vehicle such as a tire pressure monitoring system (TPMS) or other sensors wherein sensor data is sent wirelessly from an electrically isolated device to a receiver connected to the vehicle electrical system. As used herein, the term fob includes both user activated remote control devices and self-triggering wireless sensors which employ backscattering of RF to transmit commands and/or data.
RFID backscatter is the most common example of backscatter communication in use today, but backscatter can be used with other RF protocols such as Wi-Fi and Bluetooth® Low Energy (BLE). Since many vehicles are already being manufactured with Wi-Fi and BLE systems, there exists the potential to incorporate passive backscatter devices that can operate near the vehicle by harvesting these (or other) RF signals and to use the harvested energy to operate an antenna circuit to backscatter these (or other) RF signals to communicate with the vehicle to send authentication data and remote user commands, for example.
When using a vehicle-mounted transmitter to broadcast an RF signal that powers the remote device and that is then backscattered by the remote device, it would be necessary to continuously broadcast the RF while a vehicle is parked. Otherwise, the RKE functions would not be continuously available to the user. This would lead to high power consumption in the vehicle, potentially resulting in depletion of the vehicle battery to a point where engine starting could fail as well as reductions in the life of the battery. Another option would be for the key fob to use ambient RF signals in the environment such as TV or cellular signals for deriving power and for modulated backscattering that could be sensed and decoded by the vehicle. However, sufficient ambient RF is not always available, such as when the vehicle and key are in an underground parking structure or in a remote, rural area.